Using the case of an implantable medical device to broaden communication bandwidth

ABSTRACT

An improved implantable pulse generator (IPG) containing improved telemetry circuitry is disclosed. The IPG includes a telemetry coil within the conductive IPG case, not in the non-conductive header as is typical, which simplifies IPG design. The improved resonant circuit of which the coil is a part does not include a discrete tuning resistor with the coil, which tuning resistor was traditionally used to increase communication bandwidth of the coil to render it suitable for FSK telemetry. In lieu of the tuning resistor, the coil is intentionally inductively coupled to the case by positioning the coil a certain distance away from the case. Such coupling decreases the effective inductance and increases the effective series resistance in the improved resonant circuit, both of which increase the communication bandwidth. As such, suitable FSK telemetry can be achieved, even though the improved resonant circuit without the case would not on its own have suitable bandwidth.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part (CIP) of U.S. patentapplication Ser. No. 12/616,178, filed Nov. 11, 2009 (“the '178application”), to which priority is claimed and which is herebyincorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to an improved implantable medical devicethat utilizes coupling between a case and a telemetry coil instead of adiscrete resistor to achieve suitable communication bandwidth.

BACKGROUND

Implantable stimulation devices are devices that generate and deliverelectrical stimuli to body nerves and tissues for the therapy of variousbiological disorders, such as pacemakers to treat cardiac arrhythmia,defibrillators to treat cardiac fibrillation, cochlear stimulators totreat deafness, retinal stimulators to treat blindness, musclestimulators to produce coordinated limb movement, spinal cordstimulators to treat chronic pain, cortical and deep brain stimulatorsto treat motor and psychological disorders, and other neural stimulatorsto treat urinary incontinence, sleep apnea, shoulder sublaxation, etc.The description that follows will generally focus on the use of theinvention within a Spinal Cord Stimulation (SCS) system, such as thatdisclosed in U.S. Pat. No. 6,516,227. However, the present invention mayfind applicability in any implantable medical device system.

As shown in FIGS. 1A and 1B, a SCS system typically includes anImplantable Pulse Generator (IPG) 100, which includes a biocompatibledevice case 30 formed of a conductive material such as titanium forexample. The case 30 typically holds the circuitry and battery 26necessary for the IPG to function, although IPGs can also be powered viaexternal RF energy and without a battery. The IPG 100 is coupled toelectrodes 106 via one or more electrode leads (two such leads 102 and104 are shown), such that the electrodes 106 form an electrode array110. The electrodes 106 are carried on a flexible body 108, which alsohouses the individual signal wires 112 and 114 coupled to eachelectrode. In the illustrated embodiment, there are eight electrodes onlead 102, labeled E₁-E₈, and eight electrodes on lead 104, labeledE₉-E₁₆, although the number of leads and electrodes is applicationspecific and therefore can vary. The leads 102, 104 couple to the IPG100 using lead connectors 38 a and 38 b, which are fixed in anon-conductive header material 36, which can comprise an epoxy forexample.

As shown in FIG. 2, the IPG 100 typically includes an electronicsubstrate assembly 14 including a printed circuit board (PCB) 16, alongwith various electronic components 20, such as microprocessors,integrated circuits, and capacitors mounted to the PCB 16. Two coils aregenerally present in the IPG 100: a telemetry coil 13 used totransmit/receive data to/from an external controller 12; and a chargingcoil 18 for charging or recharging the IPG's battery 26 using anexternal charger (not shown). The telemetry coil 13 is typically mountedwithin the header 36 of the IPG 100 as shown, and may be wrapped arounda ferrite core 13′. Coil 13 is connected to the circuitry inside thecase 30 via feedthrough connectors 24.

As just noted, an external controller 12, such as a hand-held programmeror a clinician's programmer, is used to wirelessly send data to andreceive data from the IPG 100. For example, the external controller 12can send programming data to the IPG 100 to dictate the therapy the IPG100 will provide to the patient. Also, the external controller 12 canact as a receiver of data from the IPG 100, such as various datareporting on the IPG's status. The external controller 12, like the IPG100, also contains a PCB 70 on which electronic components 72 are placedto control operation of the external controller 12. A user interface 74similar to that used for a computer, cell phone, or other hand heldelectronic device, and including touchable buttons and a display forexample, allows a patient or clinician to operate the externalcontroller 12. The communication of data to and from the externalcontroller 12 is enabled by a coil (antenna) 17.

Wireless data telemetry between the external controller 12 and the IPG100 takes place via inductive coupling, and specifically magneticinductive coupling. To implement such functionality, both the IPG 100and the external controller 12 have coils 17 and 13 which act togetheras a pair. When data is to be sent from the external controller 12 tothe IPG 100 for example, coil 17 is energized with an alternatingcurrent (AC). Such energizing of the coil 17 to transfer data can occurusing a Frequency Shift Keying (FSK) protocol for example, in whichdigital data bits in a stream are represented by different frequencies.For example, frequency f₀ represents a logic ‘0’ (e.g., 121 kHz) andfrequency f₁ represents a logic ‘1’ (e.g., 129 kHz). Energizing the coil17 in accordance with these frequencies produces a magnetic field, whichin turn causes coil 13 in the IPG to resonate. Such resonance induces avoltage in the IPG's coil 13, which produces a corresponding currentsignal when provided a closed loop path. This voltage and/or currentsignal can then be demodulated in the IPG 100 to recover the originaldata. Transmitting data from the IPG 100 to the external controller 12occurs in essentially the same manner.

Typical communication circuitry for an IPG 100 such as that illustratedin FIGS. 1A, 1B and 2 is shown in FIG. 3A. An inductance L_(coil) of thecoil 13 and a capacitor C comprise a resonant circuit 75 that allows forboth transmission and reception of FSK data signals. Although theinductance L_(coil) and capacitance C are shown in series in resonantcircuit 75, one skilled in the art will realize that such parameters canalso be coupled in parallel. Generally, values for L_(coil) and C arechosen so that resonance happens most strongly at a center frequency,f_(c), which value is generally at the midpoint between f₀ and f₁ (e.g.,125 kHz). Coil 13 can be electrically modeled as having an inductanceL_(coil) and a self resistance, R_(self). R_(self) is the nativeresistance of the wire used to form the coil 13, and is measured at theAC operating frequency. Transceiver circuitry 54 and the microcontroller55 are well known, and do not require substantial elaboration. Oneskilled will understand that the transceiver circuitry 54 includesamplifiers, modulators, demodulators, and other circuits to in effecttranslate a serial digital data stream to and from the IPG's processmicroprocessor 55, depending on whether data reception or transmissionis occurring.

An important consideration in the design of the IPG's resonant circuit75 is it bandwidth, because the bandwidth of the resonant circuitryneeds to be wide enough to include both of the FSK frequencies f₀ andf₁. (The same is true for the matching resonant circuitry in theexternal controller 12, but because such circuitry is not the focus ofthis disclosure and can merely be the same as the circuitry in the IPG100, such external circuitry is ignored). It is well known in the art,that the bandwidth of a series resonant circuit depends upon its qualityfactor (Q). The quality factor, Q, depends on the inductance, theresistance in series with the coil, and the center frequency:

$\begin{matrix}{Q = \frac{2\pi\;{f_{c} \cdot L_{coil}}}{R}} & (1)\end{matrix}$Further, the half-power or −3 dB bandwidth of the resonant circuit isdependent on Q:

$\begin{matrix}{{BW} = \frac{2\pi\; f_{c}}{Q}} & (2)\end{matrix}$When these two equations are combined, the bandwidth can be expressedas:

$\begin{matrix}{{BW} = \frac{R}{L_{coil}}} & (3)\end{matrix}$

In prior art IPG resonant circuits 75, it was generally required tospecifically add an additional discrete resistor, R_(tune), to increasethe bandwidth to a suitable level inclusive of f₀ and f₁. This isillustrated in FIG. 3B, which shows the frequency responses whenR_(tune) is included (curve 59) and not included (curve 58) in theresonant circuit 75. When R_(tune) is not included in the circuit (curve58), the bandwidth 63 (measured at −3 db line 60) does not include FSKfrequencies f₀ or f₁, meaning that the communication would be inadequateto either transmit or receive such frequencies. By contrast, whenR_(tune) is included in the circuit (curve 59), the bandwidth 62(measured at −3 db line 61) includes FSK frequencies f₀ or f₁, meaningthat such frequencies can be transmitted or received with goodefficiency. Table 1 shows typical values for an exemplary prior artresonant circuit designed to operate at f₀=121 kHz and f₁=129 kHz with acenter frequency of f_(c)=125 kHz:

TABLE 1 Parameter Value L_(coil) 1290 μH R_(self) 26 Ω R_(tune) 100 Ω Q8 Bandwidth 15.5 kHzAs can be seen, a tuning resistor R_(tune) (100Ω) is needed which issignificantly larger than R_(self) (26Ω) to provide a suitable bandwidth(˜15 kHz) to encompass f₀=121 kHz and f₁=129 kHz around the centerfrequency f_(c)=125 kHz with suitable margin. Without R_(tune) included,the bandwidth decreases to about 3.1 kHz, which would range from about123.5 to 126.5 kHz, and hence does not reach either of f₀ or f₁.

(R_(tune) can also be added in parallel to the L_(coil) to broaden thebandwidth. However, because the value for R_(tune) in this parallelconfiguration would usually be a lot higher than were R_(tune) used inseries with L_(coil), a series connection is simpler).

The inventors consider certain aspects of the design of IPG 100 to benon-optimal. For one, the inventors find it unfortunate that thetelemetry coil 13 resides in the IPG's header 36. This requiresfeedthroughs 24 (FIG. 2) to couple the coil 13 to the other resonantcircuit 75 components and to the transceiver circuitry 54, all of whichreside inside the case 30. Such feedthroughs 24 add to the complexity ofthe design of the IPG 100, and can lead to problems with hermeticity.

Another disadvantage of having the coil 13 in the header 36 is that thecoil 13 takes up space in the header, which space is becoming morelimited at IPG technology advances. It is desirable for patient comfortto continue to make IPGs 100 smaller, which shrinks header 36 volumeaccordingly. At the same time, future-generation IPGs are expected tooffer even greater numbers of electrodes (e.g., 32, 64, etc). Butaccommodating an increased number of electrodes requires more space forlead connectors such as 38 a and 38 b (FIGS. 1A and 1B) in the header36. As such, it is anticipated by the inventors that there may be littleroom left in the header for an adequate telemetry coil 13. Moreover,because the coil 13 in the header 36 must be rather small, a ferritecore 13′ is usually beneficial to increase the magnetic flux throughcoil 13, and thus its communication efficiency. But the ferrite core 13′can potentially interfere with certain procedures, such as MagneticResonance Imaging (MRI), which limits the utility of designs using suchcores.

It is also undesirable in the inventor's opinion to have to include adiscrete tuning resistor R_(tune) to tune the bandwidth of thecommunication circuitry. Current flowing through resistor R_(tune) 53dissipates heat in the specific location of that resistor, which “hotspot” can cause the resistor to either fail or deviate from its designedvalue, either of which adversely affects the reliability of the IPG 100.Moreover, it is generally desired to minimize the number of discretecomponents such as R_(tune) in the case 30 of the IPG 100, because asjust noted it is desirable to make the IPG 100 as small as possible andspace inside the case 30 is limited.

A solution to these problems is provided in this disclosure in the formof a new mechanical and/or electrical design for an IPG, or otherimplantable medical device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show an implantable medical device, and the manner inwhich an electrode array is coupled to the IPG in accordance with theprior art.

FIG. 2 shows the relation between the implantable medical device and anexternal controller.

FIGS. 3A and 3B show the resonant circuit of the IPG of the precedingfigures and in particular a tuning resistor and its effect on thebandwidth of the telemetry coil of the IPG.

FIGS. 4A and 4B show an improved IPG in accordance with the invention,in which the telemetry coil is within the IPG case and positioned acertain distance from the IPG case.

FIG. 5 shows the resonant circuit of the IPG of FIGS. 4A and 4B, and inparticular shows that the tuning resistor of the prior art has beenreplaced by a coupling M between the coil and the case.

FIGS. 6A and 6B show equivalent circuits for the telemetry coil and thecase when the telemetry coil is within the case at a distance d from thecase.

DETAILED DESCRIPTION

The description that follows relates to use of the invention within aspinal cord stimulation (SCS) system. However, it is to be understoodthat the invention is not so limited, and could be used with any type ofimplantable medical device system.

An improved implantable pulse generator (IPG) containing improvedtelemetry circuitry is disclosed. The IPG includes a telemetry coilwithin the conductive IPG case, not in the non-conductive header as istypical, which simplifies IPG design. The improved resonant circuit ofwhich the coil is a part does not include a discrete tuning resistor inseries or in parallel with the coil, which tuning resistor wastraditionally used to increase communication bandwidth of the coil torender it suitable for FSK telemetry. In lieu of the tuning resistor,the coil is intentionally inductively coupled to the case by positioningthe coil a certain distance away from the case. Such coupling decreasesthe effective inductance and increases the effective series resistancein the improved resonant circuit, both of which increase thecommunication bandwidth. As such, suitable FSK telemetry can beachieved, even though the improved resonant circuit without the casewould not on its own have suitable bandwidth.

An improved IPG 200 is shown in FIGS. 4A and 4B. Because the mechanicalstructure of IPG 200 is already discussed at length in theabove-referenced '178 application, many of the details will not bereiterated here. In the design of IPG 200, the telemetry coil 13 isplaced inside the case 30, and is wound in a plane parallel to a planeof the case. Because the telemetry coil 13 is placed inside the case 30,and not in the header 36 as in the prior art (FIG. 2), feedthroughconnectors 24 (FIG. 2) are not required to couple the coil 13 to theremainder of the communication circuitry, which simplifies IPG design.Moreover, telemetry coil 13 is preferably made to encompass a large areaA (FIG. 4A) when compared to the smaller coil 13 used in the header 36in the prior art design. (The IPG's case 30 is removed in FIG. 4A foreasier viewing). This larger area improves coupling, and hencereliability of data transfer, with the telemetry coil 17 in the externalcontroller 12 (FIG. 2). Larger area A also compensates for the lack of aferrite core 13′ within the telemetry coil 13, which ferrite core iseliminated in the IPG 200. This again simplifies IPG design, and allowsIPG 200 to be more compatible with Magnetic Resonance Imaging (MRI)techniques. Finally, by moving the telemetry coil 13 into the case 30,more room is left in the header 36 for the lead connectors, such as leadconnectors 38 a and 38 b shown in FIG. 3B.

Changes to the mechanical design of the IPG 200 facilitate changes tothe IPG 200's resonant circuit 175, which is shown in FIG. 5. As withresonant circuit 75, the improved resonant circuit 175 is shown with thetuning capacitance C in series with L_(coil), although it could also beplaced in parallel. Notice that unlike the prior art communicationcircuitry depicted in FIG. 3A, the improved resonant circuit 175 in FIG.5 lacks a tuning resistor altogether. As such, the resonant circuit 175is simpler, and omits a discrete resistor within the case 30 where spaceis at a premium. Additionally, concern over forming a “hot spot” at thelocation of such resistor is alleviated. Also present in FIG. 5 as partof the resonant circuit 175 is the IPG case 30, which is coupled to coil13 by a coupling factor M. The relevance of the case 30 to the resonantcircuit 175 will be explained shortly.

As noted earlier, in the prior art design, the tuning resistor,R_(tune), was needed to adjust the bandwidth of the resonant circuit 75to render it suitable for FSK telemetry: without the additionalresistance of R_(tune), the bandwidth was too narrow and would notencompass FSK frequencies f₀ and f₁. The improved resonant circuit 175actually would suffer from this same bandwidth problem if treated inisolation. However, when the improved communication circuitry isproperly positioned within conductive case 30, such coupling changes theparameters of the resonant circuit 175 to suitably broaden thebandwidth.

FIG. 4B shows the improved IPG 200 in cross section, and illustrates thepositioning allowing for suitable FSK performance without the need for adiscrete R_(tune) resistor. Of specific importance is the distance, d,between the coil 13 and the conductive case 30. Traditionally in implanttechnology, it was generally desirable to isolate the coil 13 from thecase 30 to the greatest extent possible to prevent interference orcoupling between the coil 13 and the case 30, which interference couldadversely affect the reliability of FSK data communications. However, inthe improved design, distance d is intentionally made small to providecoupling to the case 30 and to broaden the communication bandwidth. Thisis counterintuitive, because as just mentioned coupling to the case canpotentially degrade the reliability of data communication. However, suchpotential degradation is minimized in other ways, such as by providing alarger area extent A of the coil 13.

Because the case 30 is conductive, the AC magnetic fields generated bythe telemetry coil 13—whether such coil is transmitting orreceiving—cause eddy currents I_(e) to flow through the conductive case30 because of inductive coupling between the two. By Lenz's law, thesecirculating eddy currents will create induced magnetic fields in thecase 30 that oppose the original magnetic field from coil 13. Theinduced eddy currents comprise power (I_(e) ²R_(case)) loses within thecase 30, where R_(case) equals the resistance of the case. Such powerloss will need to be compensated for by increasing the power draw in thetransceiver circuitry 54.

FIGS. 6A and 6B describe further the interaction between the telemetrycoil 13 and the case 30. As shown in FIG. 6A, FIG. 6A shows thetelemetry coil 13 and the conductive case 30. The telemetry coil 13 asbefore is represented by an inductance L_(coil) and a resistanceR_(self). Case 30 is represented by an inductance L_(case) and aresistance R_(case). M represents the amount of coupling between thetelemetry coil 13 and the case 30. d represents the distance between thetelemetry coil 13 and the case 30, and is inversely related to thecoupling factor, M.

An equivalent circuit for the network in FIG. 6A is shown in FIG. 6B. Inthe equivalent circuit, the inductive coupling between the case 30 andthe coil 13 can be simplified as an resistance, R_(eff), in series withan effective inductance, L_(eff). (R_(eff) generally scales withR_(case)). While one of ordinary skill in the art can represent L_(eff)and R_(eff) mathematically, it suffices here to observe that as thedistance d decreases—i.e., as coil 13 is brought closer to the case30—(1) R_(eff) will increase, and (2) L_(eff) will decrease from aninitial value of L_(coil) (i.e., L_(eff)=L_(coil) at d=∞). (Suchrelations between distance d and R_(eff) and L_(eff) are morecomplicated than a simple linear or inverse linear relationship).

Returning to bandwidth Equation 3 discussed above, notice that that theincrease in R_(eff) and the decrease in L_(eff) brought about by thecoupling between the coil 13 and the case 30 both assist in increasingthe bandwidth, BW. As such, when the bandwidth of the improved resonantcircuit 175 is compared with (Eq. 4) and without (Eq. 5) the case, itcan be seen that the former is larger (Eq. 6):

$\begin{matrix}{{BW}_{{with}\mspace{11mu}{case}} = \frac{R_{self} + R_{eff}}{L_{eff}}} & (4) \\{{BW}_{{no}\mspace{14mu}{case}} = \frac{R_{self}}{L_{coil}}} & (5) \\{{BW}_{{with}\mspace{14mu}{case}} > {BW}_{{no}\mspace{14mu}{case}}} & (6)\end{matrix}$Moreover, because the inclusion of the case 30 affects a larger changein series resistance resistance (i.e., from R_(self) toR_(self)+R_(eff)) than it does a change in the inductance (i.e., fromL_(coil) to L_(eff)), R_(eff) rather than L_(eff) tends to dominate theincrease in the bandwidth.

To further illustrate the increase in bandwidth that the case 30provides in the improved resonant circuit 175, Table 2 representsexemplary values for the resonant circuit 175 with and without the casefor a particular coil-to-case distance, d=2.5 mm, and assuming aresonant capacitor C of 5.4 nF:

TABLE 2 Parameter No Case With Case L L_(coil) = 325 μH L_(eff) = 300 μHR R_(self) = 8 Ω R_(self) + R_(eff) = 29 Ω Q 32 8 BW BW_(no case) = 3.9kHz BW_(with case) = 15.6 kHz D — 2.5 mmNote as discussed above that the change from R_(self) toR_(self)+R_(eff) (from 8 to 29Ω) is more pronounced than the change fromL_(coil) to L_(eff) (from 325 to 300 μH), bearing out the dominance ofR_(eff) in effecting changes to the bandwidth.

Regarding such bandwidth changes, note from Table 2 that resonantcircuit 175 without the case 30 would be ineffective for FSKcommunication at the frequencies noted earlier: at a bandwidth of 3.9kHz, and assuming a center frequency f_(c)=125 kHz, such circuitry couldonly reliably resolve frequencies in the range of approximately 123 to127 kHz, and so could not reliably transmit or receive communications atf₀=121 kHz or f₁=129 kHz. However, when the case 30 is included, thebandwidth increases to approximately 15.6 Hz, allowing resolution offrequencies from approximately 117 kHz to 133 kHz, which is able toresolve f₀ and f₁ with considerable margin. Although a bandwidthincrease of four times (15.6/3.9) is experienced with these conditions,other useful embodiments of the technique can be defined as the case 30contributing at least a two-times increase in bandwidth compared to theresonant circuit 175 in isolation.

In short, the case 30 in the improve resonant circuit 175 replaces thefunction of R_(tune) in the prior art resonant circuit 75 (FIG. 3A).Even if the increased effective series resistance R_(eff) grows largeand comprises a power draw, such power draw will not be limited to adiscrete location, and instead will be distributed between the coil 13and case 30, which beneficially spreads any heating in the IPG 100.

Proper tuning of the improve resonant circuit 175 requires theconsideration of several factors, including at least L_(coil), R_(self),R_(case), C, and distance d. It should be appreciated by one skilled inthe art that computerized simulations may only be moderately helpful inchoosing values for these different parameters given the complexity ofthe physics involved. For example, when choosing a particular distance dgiven fixed values for the other parameters in the resonant circuit 175,it may be advisable to build and test a prototype IPG, and tomechanically vary the distance d to see where d is optimized from abandwidth and other perspectives. Such experimentation is fortunatelyroutine for one skilled in the art, even if potentially time-consuming.

Although disclosed in the context of Frequency Shift Keying (FSK) usingonly two discrete frequencies to represent the two digital logic statesof ‘1’ and ‘0’, it should be recognized that the disclosed technique forbroadening the bandwidth is applicable to FSK techniques involving morethan two discrete frequencies. As is known, 2^(N) frequencies can alsobe used to send 2^(N) digital symbols, with each symbol comprising Nbits. For example, eight frequencies (e.g., 121, 122, 123, 124, 125,126, 127, and 128 kHz) can be used to represent eight different digitalsymbols (e.g., 000, 001, 010, 011, 100, 101, 110, and 111), and thedisclosed technique can be used to broaden the bandwidth to cover alleight frequencies. Such symbols can be considered as digital logicstates for purposes of this disclosure.

Although particular embodiments of the present invention have been shownand described, it should be understood that the above discussion is notintended to limit the present invention to these embodiments. It will beobvious to those skilled in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe present invention. Thus, the present invention is intended to coveralternatives, modifications, and equivalents that may fall within thespirit and scope of the present invention as defined by the claims.

What is claimed is:
 1. An implantable medical device, comprising: aplanar conductive case; a resonant circuit within the case fortransmitting, receiving, or transmitting and receiving data at at leasttwo frequencies each representing a digital logic state, the resonantcircuit comprising: a telemetry coil, wherein the telemetry coil iswound in a plane parallel to a plane of the case, and spaced from theplane of the case by a distance to cause coupling between the case andthe telemetry coil; and a capacitor, wherein the coupling between thecase and the telemetry coil at least doubles a bandwidth of the resonantcircuit.
 2. The device of claim 1, wherein the resonant circuit does notinclude a discrete resistor.
 3. The device of claim 1, furthercomprising a non-conductive header coupled to the conductive case. 4.The device of claim 1, further comprising at least one lead connectorwithin the non-conductive header for coupling to an electrode lead. 5.The device of claim 1, wherein the telemetry coil and the capacitor arecoupled in series.
 6. The device of claim 1, wherein the telemetry coiland the capacitor are coupled in parallel.
 7. The device of claim 1,wherein the resonant circuit does not include a discrete resistanceapart from a self resistance of the telemetry coil.
 8. An implantablemedical device, comprising: a resonant circuit which in isolation has afirst bandwidth, the resonant circuit comprising a telemetry coil and acapacitor; and a conductive case, wherein the resonant circuit ispositioned within the conductive case, wherein the resonant circuitinside the conductive case has a second bandwidth larger than the firstbandwidth, wherein the second bandwidth is at least double the firstbandwidth.
 9. The device of claim 8, wherein the resonant circuit doesnot include a discrete resistor.
 10. The device of claim 8, furthercomprising a non-conductive header coupled to the conductive case. 11.The device of claim 8, further comprising at least one lead connectorwithin the non-conductive header for coupling to an electrode lead. 12.The device of claim 8, wherein the conductive case is planar, andwherein the telemetry coil is wound in a plane parallel to a plane ofthe case, and separated from the plane of the case by a distance. 13.The device of claim 12, wherein the resonant circuit does not include adiscrete resistance apart from a self resistance of the telemetry coil.14. The device of claim 8, wherein the first bandwidth is determined byan inductance of the telemetry coil and a self resistance of thetelemetry coil.
 15. The device of claim 14, wherein the second bandwidthis determined by the effect of coupling between the telemetry coil andthe case.